The present invention relates to a method of producing same-sized particles, and, more particularly, to a method of treating oversized particles, such as generally spherical or nearly spherical semiconductor particles, so that they possess the same size, or approximately the same size, and are therefore suitable for inclusion in relevant products. In specific embodiments, the particles are silicon spheres and the products are solar cells.
One type of solar cells includes a plurality of spherical semiconductor particles or members which protrude partially through both sides of, and are affixed to the walls of apertures formed in, a first flexible metal foil sheet. The details of the construction (i.e., the mechanical and electrical form, fit and function) and fabrication methodology of this type of solar cell may be found in the following commonly assigned U.S. Pat. Nos. 5,192,400; 5,091,319; 5,086,003; 5,028,546; 4,994,878; 4,992,138; 4,957,601; 4,917,752; 4,872,607; 4,806,495; and 4,691,076.
The construction details and the fabrication methodology of the type of solar cells with which the present invention is concerned are briefly summarized below.
This fabrication methodology utilizes generally uni-sized spheres of a semiconductor material, such as silicon, each of which has a p-n junction and is produced by one of a number of available techniques. See, for example the production methods disclosed in commonly assigned U.S. Pat. Nos. 4,637,855; 5,012,619; and 5,069,740. The silicon spheres typically constitute an outer portion or shell of one conductivity type surrounding an inner silicon portion of the other conductivity type, both portions having a selected purity and other relevant characteristics. The spheres are capable of producing electricity when radiation, such as sunlight, is incident thereon. The produced electricity may flow between conductors, one of which is electrically continuous with the outer portion of each sphere, and the other of which is electrically continuous with the inner portion of each sphere. In the aforenoted patents, these conductors are preferably flexible metal foils, to the first of which the spheres are affixed, as noted above.
Typical silicon sphere production techniques tend to produce batches of intermingled silicon spheres or spheroids having varying sizes and diameters. Thus, for reasons more fully set forth below, manufacturing the above type of solar cell requires either a method of rendering the spheres the same size or a non-damaging method of sorting the fragile silicon spheres. Such sorting methods are the subject of the following commonly assigned U.S. patent applications Ser. No. 08/159,645, filed Nov. 30, 1993; Ser. No. 08/159,673, filed Nov. 30, 1993; and Ser. No. 08/159,872, filed Nov. 30, 1993. Whether uni-sized spheres are achieved by an affirmative treatment step, as is the case with the present invention, or by sorting, the manner of acquiring same-sized spheres should be efficient and have high throughput so as not to constitute a bottleneck in a solar cell manufacturing operation.
Manufacturing the foregoing type of solar cells begins with forming in the first metal foil a pattern of apertures, the diameters of which are slightly less than the diameters of an available quantity of same-sized silicon spheres or spheroids. Several methods for forming the apertures are available. After formation of the aperture pattern, the spheres are loaded onto one side of the foil so that each aperture is occupied by a sphere. Because of the relative sizes of the diameters of the same-sized spheres and the apertures, the aperture-located spheres merely nest in their respective apertures on one side of the first foil without substantially protruding from both sides thereof.
The spheres are mechanically and electrically affixed and connected to the first foil. Such affixation and connection are achieved by applying suitable compressive forces to the foil-sphere system, as set forth in the above-noted patents. Typically the application of the compressive forces is achieved by the use of a press which acts on the spheres and the foil through selected compliant and rigid elements which are positioned between working surfaces of the press and the foil-sphere system. These elements prevent damage to the spheres and to the foil, while ensuring that the applied forces effectively move the spheres partially through and affix them to their respective apertures.
Partial movement of the spheres through their respective apertures effects mechanical affixation of the spheres to the walls of their apertures and renders electrically continuous with the first foil the outer surfaces of the spheres. These ends are achieved, in part, through the relationship of the larger diameters of the spheres to the smaller diameters of the apertures. This relationship directly results in the mechanical affixation and aids in effecting electrical continuity of the spheres with the first foil. When the spheres are moved partially through the apertures, the edges of the aperture walls and the surface of the spheres mechanically interact and mutually abrade each other to remove any natural oxide on the spheres or the aperture walls. Thus, a metal-sphere (i.e., an aluminum-silicon) bond is formed. The foregoing may be enhanced by the application of heat during the compression.
The outer portion of one conductivity type of the located and affixed spheres is removed, as by etching, from the spheres. This removal occurs only on one side of the first foil to expose the inner sphere portions of the opposite conductivity type. An electrically insulative layer is applied or deposited on the exposed inner sphere portions and the one side of the first foil. Small regions of the insulative layer which overlie the exposed inner sphere portions are removed, as by abrading or etching, to create openings or vias through which access to the inner sphere portions may be obtained. A second flexible metal foil is then mechanically and electrically connected to the inner portions of the spheres through the access openings by thermocompression bonding or a functionally equivalent technique.
The solar cell is now nearly complete and constitutes a flexible, photovoltaic matrix. Radiant energy directed toward the free surface of the first foil falls on the spheres to produce electricity. A utilization device is connected between the foils. The electricity flows from one portion, inner or outer, of the spheres through one of the foils, through the utilization device and ultimately through the other foil into the other portion, outer or inner, of the spheres. The insulative layer electrically insulates the foils from each other. The flexible cell may be conformed to a desired surface or shaped in selected fashion. A protective cover may be placed over or applied to the spheres. The cover may include or comprise lenses which direct an increased amount of incident radiant energy onto the spheres to increase the efficiency of the cell.
There are numerous reasons for the silicon spheres used in the foregoing type of solar cell to be approximately the same size. Such reasons are set forth in the foregoing patents and applications. In the past, uniform sphere diameters have been achieved by manual sorting followed by grinding or abrading oversized spheres until they have the same diameter as spheres determined to be acceptable as a result of manual sorting. At least the grinding phase of a sortinggrinding methodology requires an abundance of time which may deleteriously affect the throughput of the overall solar cell manufacturing process.
An object of the present invention is the provision of an efficient, high throughput, non-damaging method of producing same-sized semiconductor spheres suitable for use in fabricating solar cells.